Broadband amplifier having uniform frequency response

ABSTRACT

A broadband amplifier includes an amplifier device and input and output networks formed of microstrips. The impedance of the output microstrip is chosen by taking the geometric mean of the real parts of one of the scattering parameters of the amplifier device, at the lower and upper limits of the frequency range of interest, and by taking the geometric mean of the system impedance with the result obtained previously. The final result of these calculations is the impedance of the output network. The impedance of the input network is made to be at least twice, and preferably twice, the impedance of the output network. The amplifier is also provided with a slope compensation circuit. It has been discovered that an amplifier designed in this manner exhibits remarkable flatness over a five-octave range. The amplifier circuit is therefore suitable for use in cable television and other broadband telecommunications applications, where the frequency range of interest may be 30 MHz to 1000 MHz.

This is a continuation of application Ser. No. 08/329,419, filed Oct.27, 1994, now U.S. Pat. No. 5,489,877.

BACKGROUND OF THE INVENTION

This invention relates to the field of broadband amplifiers, especiallythose amplifiers used in cable television and telecommunicationssystems. The amplifier circuit of the present invention has an extremelyuniform response over a very wide range of frequencies.

A cable television system requires an amplifier capable of providing adesired amount of amplification over a wide bandwidth, typically from 30MHz to 1000 MHz. It is desired that the amplifier have a gain whichremains within +/-1 dB (and preferably +/-0.5 dB) of a desired valueover the frequency range of interest. Of course, the gain ofconventional amplifiers varies with frequency. A conventional amplifier,operated alone, simply cannot achieve the desired flatness over thefive-octave frequency range specified above. In order to achieve thedesired flatness over the desired frequency range, it has been necessaryto use multiple compensation circuits and/or additional amplifiers.

One type of amplifier which is especially suitable for use in cabletelevision systems is the monolithic microwave integrated circuit(MMIC). Such devices are commercially available; a typical MMIC is theMSA-1104, sold by Hewlett-Packard. The latter MMIC is designed for useover a broad frequency band. Nevertheless, when used from 30 MHz to 1000MHz, the latter MMIC has a gain which varies by about 2.1 dB over thisrange. FIG. 4a provides a graph of gain versus frequency for this MMIC.The graph is negatively sloped, indicating that the gain of theamplifier decreases towards the upper end of the frequency range. Inorder to use the MMIC throughout the desired frequency range, it isnecessary somehow to compensate for this variation so that the frequencyresponse is essentially flat.

It has been known to compensate for a sloped frequency response curve byadding a circuit having a transfer function that is negative withrespect to that of the original amplifier. Such a compensation circuitcould comprise "lumped" elements which include capacitors, inductors,and resistors. An example of such a compensation circuit is shown withinthe dotted lines in FIG. 2. But it turns out that this slopecompensation circuit alone does not yield the desired flatness offrequency response because it presents a non-constant impedance acrossthe desired bandwidth.

The present invention provides a circuit which achieves the desiredflatness of response over the five-octave frequency range mentionedabove. The invention combines either a conventional MMIC, an MIC (i.e. amicrowave integrated circuit, in which the active device is monolithicbut is mounted on a substrate having other discrete components), or adiscrete transistor with a slope compensation circuit, and with anetwork of microstrips, so as to provide the necessary frequencycompensation.

SUMMARY OF THE INVENTION

The amplifier circuit of the present invention includes an amplifierdevice, a slope compensation circuit, and input and output networksconnected to the amplifier device. The essence of the invention residesin the selection of the impedances of the input and output networks.

To determine the impedances of the input and output networks, one firstdetermines the scattering parameter s₂₂ for the amplifier device. Theparameter s₂₂ represents the ratio of incident and reflected power atthe output port of the MMIC or transistor and is characteristic of thatparticular device. The values of parameter s₂₂, for each of severalfrequencies, are generally provided by the manufacturer of the amplifierdevice. One then obtains the real parts of the values of s₂₂ at thelowest and highest frequencies of the range within which the amplifieris to operate.

One next takes the geometric mean of the values obtained above. Then,one obtains the geometric mean of the system impedance and the resultpreviously calculated. The latter result is the impedance of the outputnetwork. The impedance of the input network should then be made at leasttwice the impedance of the output network.

Each of the input and output networks comprises a microstrip, or a setof microstrips, mounted on a circuit board and connected to the inputand output ends of the amplifier device. The impedances of eachmicrostrip can be readily selected by appropriate choice of dimensions.

While the impedance of the input network may be more than twice theimpedance of the output network, it is generally preferable to keep thevalue of the input network impedance as low as practical. Therefore, thepreferred value for the impedance of the input network is about twicethat of the output network.

The circuit made according to the procedure described above exhibits thedesired flatness of frequency response over the above-mentionedfive-octave bandwidth.

The present invention therefore has the primary object of providing anamplifier whose frequency response is essentially flat over a widebandwidth.

The invention has the further object of making it possible to use asingle amplifier over a frequency range of about 30 MHz to 1000 MHz.

The invention has the further object of providing an amplifier asdescribed above, wherein the amplifier does not need to be combined withexpensive auxiliary components, or with other amplifiers, to achieve thedesired flat response.

The invention has the further object of improving the utility andefficiency, and reducing the cost, of amplifiers used in cabletelevision and other broadband multichannel systems.

The person skilled in the art will recognize other objects andadvantages of the invention, from a reading of the following briefdescription of the drawings, the detailed description of the invention,and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic diagram of an amplifier, used forillustrating the definition of the scattering parameters associated withthe amplifier device used in the present invention.

FIG. 2 provides a schematic diagram of an amplifier of the prior art,using an RLC circuit which compensates for variation in gain of theamplifier.

FIG. 3 provides a schematic diagram of the basic structure of a circuitmade according to the present invention.

FIG. 4a provides a graph showing the gain of the monolithic microwaveintegrated circuit (MMIC) used in the present invention, over thefrequency range of interest.

FIG. 4b provides a graph showing the response curve of the compensationcircuit which is used both in the prior art and in the circuit of thepresent invention.

FIG. 4c provides a schematic diagram of a MMIC amplifier and gaincompensation circuit, as used both in the prior art and in the presentinvention.

FIG. 5 shows a Smith chart which can be used to calculate impedancevalues when performing the calculations needed to design the microstripsused in the present invention.

FIG. 6 provides a schematic diagram of a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates the terminology used in this disclosure. The block inFIG. 1 represents an amplifier, and the values A₁, A₂, B₁, and B₂represent amounts of power traveling in the directions indicated by thearrows. In general, the latter values are represented as complexnumbers. It is customary to define four "scattering parameters", alsoknown as "s parameters", as follows:

    s.sub.11 =B.sub.1 /A.sub.1

    s.sub.22 =B.sub.2 /A.sub.2

    s.sub.21 =B.sub.2 /A.sub.1

    s.sub.12 =B.sub.1 /A.sub.2

The following description refers to the scattering parameters definedabove.

In an amplifier having a flat response over a wide frequency range, onecannot use precisely tuned circuits. Instead, the designer must chooseamong several techniques that together produce a flat frequencyresponse. The present invention uses the technique of providingmicrostrip transmission lines (i.e. matching networks) that purposelymismatch the input and output impedances of the amplifier device (whichcould be a MMIC amplifier, an MIC, or a transistor), so as to compensatefor the negative slope of the frequency response (|s₂₁ |) of the device.FIG. 4a provides a graph showing the frequency response of aHewlett-Packard MSA-1104 MMIC amplifier, over the range of 30 MHzthrough 1000 MHz. For discrete transistors, the slope of the responsecurve is typically about 6 dB per octave. For MMIC amplifiers, thefrequency response is more flat, but still not sufficient over afive-octave range.

When one mismatches the input and output impedances of the amplifier,the result is an improvement in the flatness of the frequency responsecurve over a wide bandwidth. However, because the impedancecharacteristics (as represented by scattering parameters s₁₁ and s₂₂) ofthe MMIC amplifier are usually not constant over a wide bandwidth, theinput and output return loss characteristics of the amplifier will notbe optimized over the entire frequency range, when using the abovetechnique. In other words, even though the gain characteristic of theamplifier has been corrected, the input and output VSWR will vary overthe bandwidth, and will be optimum only at certain frequencies. Thelatter fact illustrates the trade-off that is made to obtain the flatfrequency response over a five-octave bandwidth.

It is possible to add more circuitry to improve the VSWR performance,but the latter approach adds to the cost of the system. It turns outthat an amplifier having the desired characteristics can be producedusing a combination of a slope compensation circuit and appropriatelychosen microstrip matching networks, as described below.

The following description outlines the procedure used for designing anamplifier system according to the present invention. In the example tobe described, the basic amplifier device is the Hewlett-Packard MSA-1104MMIC amplifier. However, the invention is not limited to this particularamplifier, nor is it necessarily limited to MMIC amplifiers. Thetechniques of the present invention can be applied to other brands andtypes of amplifiers.

As shown in FIG. 4a, the Hewlett-Packard MSA-1104 has a gain of 12.8 dBat 30 MHz and a gain of 10.7 dB at 1000 MHz. Thus, this amplifier has afrequency response curve showing a negative slope of 2.1 dB over theabove range. To equalize the frequency response over this bandwidth, oneuses a compensation circuit, such as the one shown in FIG. 4c. Note thatthe gain characteristics of the MMIC amplifier itself cannot be changedbecause they are inherent to the device. But the overall frequencyresponse of the system can be adjusted by adding circuitry havingfrequency response characteristics that mirror those of the MMIC device.This mirroring effect is accomplished by inserting a functionalattenuation curve that has the greatest effect at the low frequency (30MHz) and little or no effect at the high frequency (1000 MHz). In FIG.4b, the value of attenuation created by the slope compensation circuitat the point labeled "A dB" is equal to -2 dB. The latter attenuationsubstantially equalizes the gain difference of the MMIC between the highand low frequencies. The values of the components of the slopecompensation circuit are determined as follows:

    R=R.sub.o /2(k-1)

    C=2/(2πf.sub.r).sup.2 L

    L=(R.sub.o (b.sup.2 -1)Nk)/(4nF.sub.b b.sup.2 (k-1)

where R_(o) =characteristic impedance of the circuit

b=f_(r) /f_(b), where f_(b) is the frequency at which half theattenuation occurs, and f_(r) is the frequency of the upper limit of thedesired range,

k is defined by the equation A dB=20 log k.

As discussed above, the impedance of this circuit over the requiredbandwidth is not constant. While the slope compensation circuit alonewill indeed improve the flatness of the frequency response, its varyingimpedance characteristics will cause ripple in the frequency response.Thus, it is necessary to provide more circuitry to achieve the desiredgain flatness of +/-0.5 dB. For this purpose, one provides themicrostrip networks described below.

It has been discovered that providing an intentional mismatch betweeninput and output impedances of the MMIC amplifier makes the frequencyresponse of the overall circuit essentially flat over a bandwidth offive octaves. The following description shows the steps of the procedureused to determine the impedances of microstrips to be connected to theinput and output sides of the MMIC amplifier:

1. One first determines the values of the real parts of the values ofs₂₂ for the MMIC amplifier, at the upper and lower limits of thefrequency range of interest. The manufacturer of the MMIC generallyprovides values of all of the scattering parameters. If these values areprovided in polar coordinates, one can easily obtain the values of theirreal parts by using a Smith chart, such as is shown in FIG. 5. The Smithchart of FIG. 5 includes a plot of the values of s₂₂ for variousfrequencies. In this example, suppose that the frequency range is 50 MHzto 1000 MHz. Then the value of the real part of s₂₂ at 50 MHz is 48ohms, and the value of the real part of s₂₂ at 1000 MHz is 37 ohms.

2. One then takes the geometric mean of the two real parts obtained instep 1. In the example given, the geometric mean of 37 and 48 is 42.14.

3. One then takes the geometric mean of the result obtained in step 2and the system impedance. In this example, the circuit is to be used ina 75 ohm system. Therefore, one obtains the geometric mean of 42.14 and75, which is 56.22, or roughly 56 ohms.

4. Next, one designs input and output microstrip networks to beconnected to the amplifier device. The output microstrip network musthave an impedance equal to the result obtained in step 3. The inputmicrostrip network must have an impedance which is at least twice thatof the output impedance. In this example, the impedance of the outputmicrostrip network is 56 ohms, and the impedance of the input microstripnetwork is 112 ohms or more. Therefore, in this example, the value of112 ohms is preferred. In the specific example used, the microstripcould not support impedances greater than 125 ohms, due to thedielectric constant and the width-to-height ratio of the material. Formaterials having other values of dielectric constant and/orwidth-to-height ratio, it is possible to provide microstrips havinggreater impedances, and the invention is intended to include all suchcases.

The relationship between the optimum value of the impedance of the inputmicrostrip, and that of the output microstrip, was empiricallydetermined. It has been discovered that, by building the input andoutput microstrip networks having impedances determined according to theprocedure described above, the resulting amplifier circuit has thedesired excellent flatness characteristics, throughout the desiredfive-octave bandwidth.

The actual physical dimensions of the microstrip lines that make up thematching networks are calculated using known procedures. The dimensionsare determined, in part, by the mechanical characteristics of theprinted circuit board material, including its dielectric constant andthickness.

A basic form of the resulting amplifier circuit is shown in FIG. 3. MMICamplifier 1 is connected to microstrip 3 at its input side. The outputnetwork comprises microstrips 5 and 7, and slope compensation circuit 9.

FIG. 6 shows a more practical circuit made according to the invention,wherein all impedance calculations were based on the use of epoxy-glassprinted circuit board material (FR-4) with a thickness of 0.062 inchesand a dielectric constant of 4.7. In FIG. 6, the input microstripnetwork comprises two microstrips separated by a capacitor. Thecapacitor removes unwanted low-frequency components. The output networkincludes more microstrips, including three "T"-shaped microstrips usedto connect the amplifier circuit to portions of a slope compensationnetwork. The output network also includes several filtering capacitors.The width and length of each microstrip is indicated in the figure.

It is desirable, but not necessary, that the input and output impedancesof the amplifier device not be excessively mismatched at the outset. Inquantitative terms, the real and imaginary parts of s₁₁ and s₂₂ shouldnot differ from each other by more than about 20%. Also, if one performsthe steps outlined above with s₁₁ in the same manner as with s₂₂, thefinal geometric mean obtained from step 3 should not differ from thegeometric mean obtained from s₂₂ by more than about 20%. In the examplegiven above, when one performs the recited steps using s₁₁ instead ofs₂₂, the result of step 3 is 58 ohms, which is very close to the 56-ohmvalue obtained using s₂₂. However, in many cases, it is possible topractice the present invention successfully even if the input and outputimpedances differ from each other by more than 20%.

The invention is not limited to the example discussed above, or to anyone type of amplifier or MMIC amplifier. Many variations of theinvention are possible, as will be apparent to the reader skilled in theart. Such variations should be considered within the spirit and scope ofthe following claims.

What is claimed is:
 1. In an amplifier circuit, the circuit including aninput network, an amplifier device, and an output network, the input andoutput networks being connected to opposite sides of the amplifierdevice,the improvement wherein the input and output networks comprisemicrostrips, and wherein the input and output networks have respectiveinput and output impedances, and wherein the input network has animpedance which is at least twice the impedance of the output network,wherein the amplifier circuit exhibits nearly uniform amplification,within +/-1 dB, over a range of frequencies from 30 MHz to 1000 MHz. 2.A method of designing a broadband amplifier circuit, the amplifiercircuit being operable over a given frequency range, the amplifiercircuit including an input network, an amplifier device, and an outputnetwork, the input and output networks being connected to input andoutput ends of the amplifier device, the method comprising the stepsof:a) calculating input and output impedances of the amplifier deviceover said frequency range, and b) designing the input and outputnetworks such that the input network has an impedance equal to at leasttwice the impedance of the output network, wherein the input and outputnetworks comprise microstrips, and wherein the designing step isperformed by choosing dimensions of the microstrips, wherein theamplifier has a gain which remains within +/-1 dB over a range of 30 MHzthrough 1000 MHz.